Multi-loop antenna for radio frequency identification (RFID) communication

ABSTRACT

A multi-loop antenna is described having a plurality of conductive loops to produce an electromagnetic field for radio frequency identification (RFID) communication with RFID tags. The conductive loops are spaced apart at least a distance that is selected based on a dimension of the RFID tags with which the antenna communicates. In this manner, the loops are positioned and spaced in a manner that reduces the size of the holes within the resulting magnetic field. In addition, the configuration of the described dual-loop antenna increases the coverage of the antenna, and decreases inter-winding capacitance, thereby increasing overall read range achieved by the antenna.

TECHNICAL FIELD

[0001] The invention relates to radio frequency identification (RFID)systems for article management.

BACKGROUND

[0002] Radio-Frequency Identification (RFID) technology has becomewidely used in virtually every industry, including transportation,manufacturing, waste management, postal tracking, airline baggagereconciliation, and highway toll management. A typical RFID systemincludes a plurality of RFID tags, at least one RFID reader or detectionsystem having an antenna for communication with the RFID tags, and acomputing device to control the RFID reader. The RFID reader includes atransmitter that may provide energy or information to the tags, and areceiver to receive identity and other information from the tags. Thecomputing device processes the information obtained by the RFID reader.

[0003] In general, the information received from an RFID tag is specificto the particular application, but often provides an identification foran article to which the tag is fixed. Exemplary articles includemanufactured items, books, files, animals or individuals, or virtuallyany other tangible articles. Additional information may also be providedfor the article. The tag may be used during a manufacturing process, forexample, to indicate a paint color of an automobile chassis duringmanufacturing or other useful information.

[0004] The transmitter of the RFID reader outputs RF signals through theantenna to create an electromagnetic field that enables the tags toreturn an RF signal carrying the information. In some configurations,the transmitter initiates communication, and makes use of an amplifierto drive the antenna with a modulated output signal to communicate withthe RFID tag. In other configurations, the RFID tag receives acontinuous wave signal from the RFID reader and initiates communicationby responding immediately with its information.

[0005] A conventional tag may be an “active” tag that includes aninternal power source, or a “passive” tag that is energized by the fieldcreated by the RFID reader. In either case, the tags communicate using apre-defined protocol, allowing the RFID reader to receive informationfrom one or more tags. The computing device serves as an informationmanagement system by receiving the information from the RFID reader andperforming some action, such as updating a database. In addition, thecomputing device may serve as a mechanism for programming data into thetags via the transmitter.

[0006] Conventional antennas for RFID readers have a single inductiveloop and operate in a relatively high frequency range, e.g., 3 megahertz(MHz) to 30 MHz. Consequently, these antennas tend to create magneticfields that suffer from “holes,” i.e., regions in which an RFID tagcannot be read even though the RFID tag is located relatively near theantenna. For example, depending on the orientation and location of thearticle to which the RFID tag is affixed, in some situations the RFIDtag may be centered above a single turn of the inductive loop of theantenna during interrogation. In this situation, substantially equalcurrent may be imposed on opposite sides of the RFID tag, which leads toa cancellation effect. As a result, the RFID tag may not be able toachieve RFID communication with the reader.

[0007] In addition, conventional antennas used with desktop RFID readerstend to create magnetic fields that extend horizontally beyond the edgesof the antennas. Consequently, articles placed proximate the antenna,e.g., next to the antenna on the desktop, may be inadvertently read bythe reader, which can lead to undesired results. For example, booksassociated with one library patron and located next to an antenna in alibrary management system may be inadvertently checked out to anotherpatron.

SUMMARY

[0008] In general, a field-shaping antenna and shielding component aredescribed that shape the magnetic field into a desirable configurationfor use in an RFID system. More specifically, a dual-loop antenna isdescribed in which the loops are positioned and spaced in a manner thatreduces the size of the holes within the resulting magnetic field. Inaddition, the configuration of the described dual-loop antenna achievesincreased field size relative to a single loop antenna with equivalentpower and decreases inter-winding capacitance, thereby increasingoverall read range achieved by the antenna.

[0009] In addition, a conductive shield is described that furtherrefines and shapes the magnetic field produced by the antenna. Forexample, the antenna may be positioned substantially horizontally on adesktop or countertop. The conductive shield may be oriented parallel tothe plane of the antenna, including located in the same plane as theantenna, and generally surrounding the antenna to limit the extent towhich the electromagnetic field extends horizontally beyond the edges ofthe antenna. As a result, an electromagnetic field is produced thatgenerally projects above and below the antenna, thus defining agenerally vertical communication zone in which RFID tags can be read.

[0010] In one embodiment, a multi-loop antenna comprises a plurality ofconductive loops to produce an electromagnetic field for radio frequencyidentification (RFID) communication with RFID tags. The conductive loopsare spaced apart at least a distance that is selected based on adimension of the RFID tags with which the antenna communicates.

[0011] In another embodiment, a radio frequency identification (RFID)system comprises an RFID tag associated with an article, and an antennahaving a plurality of conductive loops to produce an electromagneticfield for communication with the RFID tag. The conductive loops arespaced at least a distance that is selected based at least in part on adimension of the RFID tag.

[0012] In another embodiment, a radio frequency identification (RFID)system comprises an antenna that forms an electromagnetic field forcommunication with RFID tags, wherein the antenna has a substantiallyplanar form. A substantially-contiguous conductive shield is positionedaround the antenna and within a plane parallel to the antenna.

[0013] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a block diagram illustrating an exemplary RFID system 2that incorporates the techniques described herein.

[0015]FIG. 2 is a block diagram that further illustrates on embodimentof an antenna of the RFID system of FIG. 1.

[0016]FIG. 3 is a plan view of an exemplary dual-loop antenna.

[0017]FIG. 4 is an exploded view of the dual-loop antenna of FIG. 3.

[0018]FIG. 5 is a schematic diagram illustrating a dual-loop antennautilized in conjunction with a conductive shield to further refine andshape the resultant magnetic field.

[0019]FIG. 6 is a side view perspective diagram illustrating exemplaryeffects on a magnetic field from a conductive shield on a single loopantenna.

[0020]FIG. 7 is another side view perspective diagram illustratingexemplary field-shaping effects of a conductive shield.

[0021]FIG. 8A is a perspective diagram illustrating a side views of anembodiment in which a conductive shield and an antenna are mounted belowa working surface.

[0022]FIG. 8B is a perspective diagram illustrating a side views of anembodiment in which an antenna is mounted in a recessed portion of aworking surface, and a conductive shield is mounted in a non-recessedportion of the working surface.

DETAILED DESCRIPTION

[0023]FIG. 1 is a block diagram illustrating an exemplary RFID system 2that incorporates the techniques described herein. In the illustratedexample of FIG. 1, RFID system 2 is used to track books, document, filesor other articles. The RFID system may, for example, be deployed withinlibraries, law offices, government agencies, or other facilities thatgenerate and/or store documents and files, such as business, criminal,and medical records. The articles contain RFID tags that uniquelyidentify the articles. In addition, each RFID tag may also containinformation describing the article, and status information indicatingwhether removal of the article is authorized. The RFID tags may beembedded within the articles so that the tags are substantiallyimperceptible, thereby reducing or prevent tampering.

[0024] In general, RFID system 2 operates within a frequency range ofthe electromagnetic spectrum, such as 13.56 MHz, with an allowablefrequency variance of +/−7 kHz. However, other frequencies may be usedfor RFID applications, and the invention is not so limited. For example,some RFID systems in large storage areas such as a warehouse may use anRFID system that operates at approximately 900 MHz.

[0025] As illustrated in FIG. 1, system 2 includes an exit controlsystem 5 that detects unauthorized removal of articles from a protectedarea. For example, the protected area may be a library and the articlesmay be books or other articles that are generally checked out from andback into the library. The techniques could also be applied to otherkinds of articles without departing from the scope of the presentinvention.

[0026] Exit control system 5 includes lattices 9A and 9B which define aninterrogation zone or corridor located near the exit of protected area.The lattices 9A and 9B include antennas for interrogating the RFID tagsas they pass through the corridor to determine whether removal of theitem to which the tag is attached is authorized. Exit control system 5may utilize at least one RFID reader (not shown) to drive the antennas.To detect a tag, the RF reader outputs RF power through the antennas tocreate an electromagnetic field within the interrogation corridor. Ingeneral, the terms “electromagnetic field” and “magnetic field” are usedinterchangeably herein as the magnetic component is used to couple withthe RFID tags.

[0027] The RF reader receives information from any tags present withinthe interrogation corridor, and exit control system 5 determines whetherremoval of the article is authorized. If removal of the article is notauthorized, exit control system 5 initiates some appropriate securityaction, such as sounding an audible alarm, locking an exit gate, etc.

[0028] In addition, RFID system 2 includes a check-in/check-out area 11by which an authorized person, e.g., a library patron or staff member,processes articles for removal or return. In particular,check-in/check-out area 11 includes an RFID reader 18 for interrogatingRFID tags fixed to articles and changing their status as desired, e.g.,checking-in or checking-out the articles.

[0029] In addition, articles may be positioned in a number of storageareas 12, e.g., on an open shelf 12A, a cabinet 12B, a vertical fileseparator 12C or a other location, as shown in FIG. 1. Each smartstorage area 12 includes tag interrogation capability which enablestracking of articles throughout a facility. In a library setting, forexample, a book could be tracked after check-in while on shelf 12A.

[0030] The RFID tags themselves may take any number of forms withoutdeparting from the scope of the present invention. Examples ofcommercially available RFID tags include 3M™ RFID tags available from 3MCompany, St. Paul, Minn., or “Tag-it” RFID transponders available fromTexas Instruments, Dallas, Tex. An RFID tag typically includes anintegrated circuit operatively connected to an antenna that receives RFenergy from a source and backscatters RF energy in a manner well knownin the art. The RFID tag modulates the RF energy providing abackscattered signal to communicate information about the RFID tag andits associated article.

[0031] An article management system 14 provides a centralized databaseof the tag information for each article in the facility. Articlemanagement system 14 may be networked or otherwise coupled to one ormore computers so that individuals, such as a librarian, at variouslocations, can access data relative to those items. For example, a usermay request the location and status of a particular article, such as abook. Article management system 14 may retrieve the article informationfrom a database, and report to the user the last location at which thearticle was located within one of the smart storage areas. Optionally,article management system 14 can re-poll or otherwise re-acquire thecurrent location of an article to verify that the article is in thelocation indicated in the database.

[0032] As described in further detail below, RFID system 2 incorporatesthe techniques described herein. Check-in/check-out area 11 and RFIDreader 18, for example, may incorporate a field-shaping dual-loopantenna 13 and a conductive shield 16 that produce a magnetic field in adesirable configuration. For example, RFID reader 18 may incorporatedual-loop antenna 13 described herein in which the loops are positionedand spaced in a manner that reduces the size of the holes within theresulting magnetic field. In addition, the configuration of thedescribed dual-loop antenna 13 achieves increased field size relative toa single loop antenna with equivalent power and decreases inter-windingcapacitance, thereby increasing overall read range achieved by RFIDreader 18.

[0033] In addition, check-in/check-out area 11 may utilize a conductiveshield 16 to further refine and shape the magnetic field produced byantenna 13. For example, as illustrated, antenna 13 may be mountedsubstantially horizontally on, within, or below desktop 15. Conductiveshield 16 may be located planar to and generally surrounding antenna 13to prevent the electromagnetic field from extending horizontally beyondthe edges of the antenna. As a result, an electromagnetic field isproduced that generally projects above and below antenna 13, thusdefining a generally vertical communication zone in which RFID tags canbe read. Conductive shield 16 may be mounted on desktop 15, or below orwithin the desktop out of view from library patrons and staff.Conductive shield 16 need not necessarily be electrically grounded toshape the magnetic field as described herein.

[0034]FIG. 2 is a block diagram that further illustrates antenna 13. Asillustrated, antenna 13 generally includes dual loops 20 that, asdescribed in further detail below, are positioned and spaced in a mannerthat reduces the size of the holes within the resulting magnetic fieldand achieves increased field size and strength. Although discussedgenerally as having dual loops, antenna 13 may have additional loopsthat are spaced based on the desired size of the tag communication zoneas well as the dimensions of the individual tags.

[0035] Tuning circuit 22 tunes dual loops 20 to a resonant frequency,and provides impedance matching and signal conversion between the loopstructure and cable 26, which may be a co-axial cable. Reader 18 iscoupled to tuning circuit 22 via cable 26 and utilizes antenna 13 forboth RFID transmit and receive operations. Consequently, reader 18 mayinclude a directional coupler to interpret the signal returned fromtuning circuit 22.

[0036]FIG. 3 is a plan view of an exemplary dual-loop antenna 30. In oneexemplary embodiment, dual-loop antenna 30 includes an inner loop 32 andan outer loop 34 that reside on parallel layers of a printed circuitboard. In another embodiment, inner loop 32 and outer loop 32 reside ina co-planar relationship.

[0037] Due to the configuration of dual-loop antenna 30, current (I)from reader 18 (FIGS. 1, 2) flows through each conductive edge of loops32, 34 in the same direction. As a result, the electromagnetic fieldscreated by the parallel conductive edges of loops 32, 34 are additive innature and achieve a resultant field having an increased field sizerelative to a single loop antenna with equivalent power.

[0038] In addition, inner loop 32 and outer loop 34 are positioned andspaced so as to reduce the number and/or size of any potential holeswithin the resultant magnetic field. For example, unlike conventionalsingle-loop antennas, reader 18 may be able to achieve successfulcommunication with an RFID tag positioned directly above a conductiveedge of the antenna. More specifically, in this situation a conventionalsingle-loop RFID antenna may produce substantially equal current onopposite sides of the RFID tag, which leads to a cancellation effect. Incontrast, an RFID tag centered above an edge of outer loop 34, forexample, will achieve increased current on the inner side of the RFIDtag due to inner loop 32. Similarly, an RFID tag centered above an edgeof inner loop 32, for example, will achieve increased current on theouter side of the RFID tag due to outer loop 34. In either case, theincreased current achieves increased energy within the RFID tag,allowing the RFID tag to successfully communicate with RFID reader 18.In this manner, the described configuration of dual-loop antenna 30 mayreduce the number and / or size of any holes within the resultantelectromagnetic field.

[0039] In one embodiment, inner loop 32 and outer loop 34 may bepositioned at least a distance D apart, where D is selected based on adimension of an RFID tag for use within the system. For example, sizesfor many conventional 13.56 MHz RFID tags range in dimension from0.5″×1″” (1.27 cm×2.54 cm) to 2″×3″” (5.08 cm×7.62 cm). Thus, in oneembodiment D may be selected to exceed a maximum dimension of the RFIDtag to ensure that no RFID tag can be positioned across both of innerloop 32 and outer loop 34, which may be advantageous in increasing theability of reader 18 to achieve successful communication with the tagsregardless of tag location. Consequently, in one embodiment D≧2.54 cm.In another embodiment, D≧5.08 cm.

[0040] Although illustrated for exemplary purposes with respect togenerally rectangular dual-loops, other forms of loops may readily beused, such as round, oval or other geometric configurations.

[0041]FIG. 4 is an exploded view of antenna 30 of FIG. 3. As describedabove, antenna 30 comprises a first layer 40 that contains inner loop 32and a second layer 42 that contains outer loop 34. Layers 40, 42 may,for example, be layers stacked on top of one another to form amulti-layered printed circuit board.

[0042]FIG. 5 is a schematic diagram illustrating a dual-loop antenna 60utilized in conjunction with a conductive shield 66 to further refineand shape the resultant magnetic field. Although illustrated forexemplary purposes with respect to a dual-loop antenna, conductiveshield 66 may be used with other forms of antennas, such as single ormulti-loop antennas of square, round or other configurations.

[0043] Conductive shield 66 may be viewed as four conductive planarregions 65A-65D that form a nearly contiguous conductive shield having anon-shielded inner region 61 around antenna 60. Conductive shield 66prevents passage of an electromagnetic field, thereby limiting themagnetic field created by antenna 60 to the inner region. In otherwords, the magnetic field created by antenna 60 extends vertically(e.g., inward and outward from FIG. 6) within inner region 61, but isprevented from forming substantially over conductive shield 66 due tothe conductive nature of the conductive shield.

[0044] Conductive shield 66 includes a disconnect area 63 that preventsa closed loop from being formed around antenna 60, thereby preventingcurrent from forming within the conductive shield. In general,disconnect area 63 may have a gap of a minimum distance D4 sufficient tocreate an electrical disconnect within conductive shield 66 and notsubstantially reduce the shielding effect of the conductive shield. Forexample, conductive shield 66 may be conventional copper or otherconductive shielding, and distance D4 need not be more than a fewmillimeters.

[0045] In general, conductive shield 66 is located a distance D3 fromouter loop 64, and the distance D3 therefore defines the outer-mostregions of the tag communication zone created by antenna 60. In otherwords, D3 defines the outermost limits of non-shielded inner region 61in which the tags may be read when antenna 60 is driven with sufficientpower to generate a magnetic field having sufficient strength to achievesuccessful communication throughout the inner region.

[0046] Each conductive regions 65A-65D has a width of D5, whichgenerally is determined based on the strength of the magnetic fieldformed by antenna 60. For example, the width D5 of each conductiveregions 65A-65D must be sufficient that the field strength at any regionbeyond, e.g., outside, of conductive shield 66 is below a thresholdlevel necessary for RFID communication. In this manner, conductiveshield 66 substantially prevents RFID communication in areas aboveconductive shield 66 until the field itself has reached a reduced fieldstrength insufficient for RFID communication, which may be at any pointbetween the inner edges and the outer edges of conductive regions 65.Consequently, D5 may be viewed as a minimum width of conductive regions65, and the conductive regions may have greater widths. For example,conductive regions 65 may be extended beyond the distance D5 for otherreasons, e.g., manufacturing simplicity. Moreover, conductive regions 65need not be of uniform widths, but rather each should preferably exceedthe minimum distance D5.

[0047]FIG. 6 is a side view perspective diagram illustrating the effectson a magnetic field from a conductive shield for which a left portion 70and a right portion 72 are depicted. For simplicity, a single-loopantenna is illustrated in FIG. 6 by conductive traces 74 and 76. Itshould be realized that with respect to the effects of a conductiveshield, a dual-loop antenna may be logically viewed as a single loopantenna having a radius equal to an average between the radii associatedwith the dual loops.

[0048] As illustrated in FIG. 6, current I within conductive traces 74and 76 create respective magnetic fields 82 and 84. Notably, magneticfields 82, 84 would extend to regions 78, 80, respectively, but for theshielding affects of left portion 70 and right portion 72, respectively.Thus, it should be realized that locating left portion 70 and rightportion 72 nearer to conductive traces 74 and 76 would further limit theoutward extent to which the resultant magnetic field is formed. Inaddition, locating left portion 70 and right portion 72 nearer toconductive traces 74 and 76 would further limit the extent to withfields 82, 84 extend inward to the opposite conductive trace. Theoverall communication zone for this single loop antenna is theapproximate sum of the magnetic fields 82 and 84.

[0049] For this reason, D3 (FIG. 5) is selected to exceed a minimumdistance necessary for the magnetic fields 82, 84 (FIG. 6) to overlap soas to ensure that a field strength is achieved within the loopssufficient for RFID communication.

[0050] In one embodiment, for example, D3 is selected to approximatelyequal the average of D1 and D2 as follows:

D3≧(D1+D2)/2.  (1)

[0051] In addition, D2 is selected to equal approximately 1.5*D1. Forexample, D1, D2 and D3 may equal 2″ (5.08 cm), 3.5″ (8.89 cm), and 2.75″(6.98 cm) respectively. This particular selection for distance D3 allowsthe resultant magnetic field created by inner loop 62 and outer loop 64(FIG. 5) to extend from these loops both in the inward and outwarddirections to entirely cover antennae 60 with sufficient strength toachieve RFID communication.

[0052]FIG. 7 is another side view perspective diagram illustrating thefield-shaping effects of a conductive shield. In particular FIG. 7illustrates a resultant electromagnetic field 90 produced by antenna 94and shaped by a conductive shield, of which a left portion 92A and aright portion 92B are depicted. As illustrated, the conductive shieldlimits the extent to which electromagnetic field 90 outwardly extendsfrom antenna 94, thereby preventing inadvertent reading of RFID tagslocated beyond the horizontal edges of a defined communication zone.

[0053]FIG. 8A is a perspective diagram illustrating a side view of oneembodiment of a check-in/check-out area 100 in which an antenna 102 andconductive shield 104 are mounted below a surface 106. In this example,antenna 102 and conductive shield 104 create an RFID tag communicationzone 107 above surface 106. Surface 106 may include visual indiciaidentifying the edges of the communication zone. In this manner,conductive shield 104 prevents inadvertent reading of RFID tags in areas108 beyond the defined communication zone 107.

[0054]FIG. 8B is a perspective diagram illustrating a side view ofanother embodiment of a check-in/check-out area 110. In this example,desktop 116 forms a recess 120, below which antenna 112 is mounted.Conductive shield 114 is mounted to surround antenna 112, on thenon-recessed portion of desktop 116. In this example, antenna 112 andconductive shield 114 create an RFID tag communication zone 117, and theconductive shield prevents inadvertent reading of RFID tags in areas 118beyond the defined communication zone. In another embodiment, desktop116 does not form recess 120, and antenna 112 is mounted below thedesktop.

[0055] Various embodiments of the invention have been described. Theseand other embodiments are within the scope of the following claims.

1. An antenna comprising a plurality of conductive loops to produce anelectromagnetic field for radio frequency identification (RFID)communication with RFID tags, wherein the conductive loops are spacedapart at least a distance D that is selected based on a dimension of theRFID tags with which the antenna communicates.
 2. The antenna of claim1, wherein the distance D is selected to exceed a maximum dimension ofthe RFID tags.
 3. The antenna of claim 1, wherein the RFID tags have adimension of length M, and the distance D between each of the pluralityof conductive loops is selected such that D≧M.
 4. The antenna of claim1, wherein D≧2.54 cm.
 5. The antenna of claim 1, wherein D≧5.08 cm. 6.The antenna of claim 1, wherein the plurality of conductive loops form adual-loop structure having an inner loop and an outer loop.
 7. Theantenna of claim 1, wherein the plurality of conductive loops areelectrically coupled so that a common current flows through the loops.8. The antenna of claim 7, wherein the plurality of conductive loops arelocated in parallel planes and formed with concentric traces, and theplurality of conductive loops are electrically coupled so that thecommon current flows through the loops in the same direction.
 9. Theantenna of claim 1, wherein the plurality of conductive loops are formedin a single printed circuit board.
 10. The antenna of claim 1, furthercomprising a tuning circuit for tuning the plurality of loops to asingle operating frequency.
 11. The antenna of claim 10, wherein thetuning circuit tunes the plurality of antennas to the operatingfrequency of approximately 13.56 megahertz (MHz).
 12. A radio frequencyidentification (RFID) system comprising: an RFID tag associated with anarticle; and an antenna having a plurality of conductive loops toproduce an electromagnetic field for communication with the RFID tag,wherein the conductive loops are spaced at least a distance that isselected based at least in part on a dimension of the RFID tag.
 13. TheRFID system of claim 12, further comprising: an RFID interrogationdevice coupled to the antenna, wherein the interrogation deviceinterrogates the RFID tag to obtain information regarding the article;and a computing device to process the information retrieved from theRFID interrogation device.
 14. The RFID system of claim 12, wherein theplurality of conductive loops are electrically coupled so that theinterrogation device drives a common current through the loops.
 15. TheRFID system of claim 14, wherein the plurality of conductive loops areformed with concentric traces, and the plurality of conductive loops areelectrically coupled so that the common current flows through the loopsin the same direction.
 16. The RFID system of claim 12, wherein each ofthe conductive loops are spaced at least a distance D that is selectedto meet or exceed a maximum dimension of the RFID tag.
 17. The RFIDsystem of claim 12, wherein the RFID tag has a dimension of length M,and the distance D between each of the plurality of conductive loops isselected such that D≧M.
 18. The RFID system of claim 17, wherein D≧2.54cm.
 19. The RFID system of claim 17, wherein D≧5.08 cm.
 20. The RFIDsystem of claim 12, wherein the plurality of conductive loops form adual-loop structure having an inner loop and an outer loop.
 21. The RFIDsystem of claim 12, wherein the antenna has a substantially planar form.22. The RFID system of claim 21, further comprising asubstantially-contiguous conductive shield positioned around the antennaand within a plane parallel to the antenna.
 23. The RFID system of claim21, wherein the conductive shield shapes the electromagnetic field toextend substantially in a direction perpendicular to the antenna, andprevents the electromagnetic field from forming substantially over theconductive shield.
 24. The RFID system of claim 23, wherein theconductive shield comprises planar conductive regions oriented to form anon-shielded inner region, and further wherein the antenna is disposedwithin the non-shielded inner region and parallel to the planarconductive regions.